The present invention relates to a method for characterising the structure of a medium and to a device for the application of this method. It is applied more particularly in the medical field for characterising human body tissues by means of ultrasonic investigation processes, but it may have an application for all other kinds of media. As a matter of fact, the characterisation of the media referred in the present invention reveals the fact that these media are able to absorb ultrasonic vibrations to a greater or lesser extent as a function of the frequency of the ultrasonic excitation beam. The principle of the measurements which then permit establishing the absorption coefficient of a medium consists in transmitting an ultrasonic signal towards this medium and in measuring the ultrasonic signal reflected by this medium and to then derive the absorption coefficient therefrom by comparing the signal transmitted and the signal reflected.
A review of the prior art is given in the paper of Messrs. P. A. Narayana and J. Ophir in the periodical "Ultrasonic Imaging" 5, 17-21, of 1983. This refers to the absorption coefficient .alpha. of a medium of which the magnitude is expressed in dB per cm and per megahertz (dB/cm/MHz). This coefficient .alpha. demonstrates that, for example, an ultrasonic beam transmitted at 10 MHz and which had penetrated into the medium to a depth of 5 cms (10 cms there and back) will have undergone an attenuation of 100 .alpha.dB (100 dB if .alpha.=1) on re-emerging from the medium. As recalled by the paper quoted, the coefficient .alpha. is related to the frequency of the excitation beam by the relationship .alpha.(f)=.alpha..sub.0 f.sup.b. In practice, b assumes a value comprised between 1 and 2 for all the media and for biological media in particular.
This paper equally demonstrates that if the Fourier spectrum of the excitation signal is gaussian about a mean frequency f.sub.0, the spectrum of the reflected signal is equally gaussian but is centred around a central frequency f.sub.c which also depends on the distance separating the surface of the medium through which the ultrasonic beam had entered from the zone of this medium which had reflected the signal. Allowing for a comparatively constant speed of propagation in the medium (1540 m/sec for media charged with water) a signal reflected by a zone of the medium re-appears at the surface of this medium at the end of a period of a longer or shorter duration depending on the greater or lesser distance at which this zone is situated from this surface.
In this way, if two zones of a medium are considered, being zones 1 and 2, an ultrasonic excitation having a gaussian spectrum and reflected by each of these zones will appear again at the surface of the medium at different instants t.sub.1 and t.sub.2. Apart from appearing at different instants, these reflected signals have different characteristics as regards amplitude and frequency spectrum. The amplitude of the signal reflected by the farthest zone, for example the section 2, is smaller than the amplitude of the signal reflected in the closest zone since this oscillation had described a longer outward and return trajectory within the medium than the latter. For a given excitation pulse, the signal received consequently decreases with time. Furthermore, the Fourier spectra of these two reflected signals are different because, since the absorption factor depends on the frequency, the high frequencies of each of their spectra undergo an attenuation as compared to their low frequencies which is the greater the longer its persistence, that is to say the farther the zone of origin of the reflected signal is situated from the surface. What this paper confirms, in the final analysis, is that despite this attenuation incurred as a function of the frequency, the spectra of the reflected signals are equally gaussian, the first around a central frequency f.sub.1 and the second around a central frequency f.sub.2, with f.sub.1 being greater than f.sub.2. If .sigma. denotes the typical difference of the gaussian distribution of the spectrum transmitted, it may then be stated that the absorption coefficient at a point of the medium situated between the zones 1 and 2 will have as its value: ##EQU1## In this expression, d.sub.1 and d.sub.2 are respectively the abscissae measured along the axis of propagation and reflection of the zones 1 and 2 with respect to the surface of the medium through which the ultrasonic oscillations enter and re-emerge. In other words, to determine .alpha. at a given point of the medium, f.sub.1 and f.sub.2 should be measured at abscissae d.sub.1 and d.sub.2 which are smaller and larger, respectively, than the abscissa of the point in question. The selection of the reflected signals RD1 (t) and RD2 (t) corresponding to the zones having abscissae d.sub.1 and d.sub.2 may be obtained by opening reception time windows at instants t.sub.1 and t.sub.2 respectively, which are related with these abscissae and with the speed of propagation c by a relationship equivalent to: ##EQU2##
The reflected signals RD1 (t) and RD2 (t) then comprise the useful data for the central frequencies f.sub.1 and f.sub.2 in their Fourier spectrum. For the sake of a clearer understanding, reference may profitably be made to the mathematical developments described in the paper quoted and in particular to formulae Nos. 11 and 17 from which had been derived the preceding expression for .alpha. as a function of f.sub.2 and f.sub.1. It will be noted in this respect that the frequency differences f.sub.2 -f.sub.0 which amount to quantities of the order of 250 KHz in many cases, comply perfectly, in respect of an ultrasonic oscillation transmitted at 10 MHz, with the supplemental condition for validation of the theory which stipulates that the relative shift from the mean transmission frequency should be lower than 20% (250/10,000 lower than 20%).
Until the present invention, it was known to measure f.sub.1 and f.sub.2 by calculating the complete spectrum RD1 (f) and RD2 (f) of each of the signals reflected by means of devices performing a fast Fourier transformation of the signal (abbreviated to FFT). As a matter of fact, these FFT devices perform a discrete Fourier transformation of the signal. This means that for each signal reflected, RD1 (t) or RD2 (t), the signal received is demodulated by means of two oscillators in phase quadrature, each of the two signals demodulated in this manner is filtered by means of a low-pass filter and quantified samples are taken by means of a blocking sampler followed by an analog/digital converter at the rate of a sampling frequency. The FFT devices thus collect a plurality of successive samples and, after a calculation period, supply a set of digital values representing the amplitude of each of the lines of the spectrum of the signal investigated. Calculations in respect of mean value and typical spectrum shift are then performed on these line amplitudes.
These FFT devices however have a first disadvantage which is related to their calculator circuit. As a matter of fact, these can operate only on a number of samples which is a power of 2: for example 64 or 128 samples. In so far as it is understandable that the accuracy of the results obtained by this method increases in direct proportion to the number of samples, it is no less understandable that this method implies assumptions regarding the stationary nature of the medium investigated. If this medium is movable, it is necessary to consider the same stationary for a limited period only, which leads to a reduction of the number of samples to be taken into account for a given measurement precision. If, making allowance for the stationary state, the optimum number of samples is of the order of 90 for example, it is observed that the measurement by means of FFT devices presents disadvantages since 90 is not a power of 2.
Furthermore, the reflected signal being affected by noise, the results secured by the FFT method are impaired by mensuration faults. Finally, these FFT devices require the application of many electronic functions and the implementation of a great number of operations. Furthermore, the need to acquire all the samples prior to any calculation, militates against real time applications in which all the points of a medium are scanned in turn and in which it is a requirement to ascertain the absorption coefficient of the different points as and when they are scanned. These real time applications are sought after in particular when it is wished to plot a graph for the factor .alpha. by means of display devices, for example on a television monitor.